As one example of optical scanning apparatuses used for a laser beam printer, etc., conventionally known is an optical scanning apparatus including a semiconductor laser, a collimator lens and a rotary polygon mirror disposed in a straight line and further including a converging lens and a photosensitive material disposed on other straight line
In this conventional optical scanning apparatus, the semiconductor laser is a laser beam source, and the laser beam source emits a laser beam towards the collimator lens. The collimator lens is located about halfway between the semiconductor laser and the rotary polygon mirror, and serves to convert the laser beam emitted from the semiconductor laser into parallel rays. The rotary polygon mirror is rotated in one direction, and a laser beam reflected on one surface of the polygon mirror is subjected to deflection scanning by the rotation thereof so as to be guided to the converging lens. The converging lens has f.multidot..theta. characteristics, and functions not only to converge a laser beam into an extremely small spot to irradiate therewith a photosensitive material of a drum shape made of a photoconductive material but also to convert the deflection scanning at a predetermined rotational speed into a linear scanning at a predetermined linear speed. Thus converted laser beam raster-scans the rotating photosensitive material to form a two-dimensional electrostatic latent image thereon. A deflection mirror is disposed on the scanning starting position side between the converging lens and the photosensitive material to guide a laser beam going to the photosensitive material through the converging lens to a synchronous detector. The synchronous detector serves to adjust timing of image formation.
U.S. Pat. No. 4,847,492 given to Youji Houki in Jul. 11, 1989, discloses an optical beam scanner including a deflection means having a mirror surface on which an optical beam emitted from an optical light source is applied, the beam being deflected by rotating the mirror surface about an axis of the deflection means; an f.multidot..theta. lens for performing an scanning operation at a uniform speed on a scanning surface; an optical compensation means for compensating for an inclination of the mirror surface of the deflection means, the optical compensation means including a cylinder lens or a cylinder mirror disposed in a space between the f.multidot..theta. lens and the scanning surface; and a detection means for receiving the deflected optical beam and detecting a scanning starting position of the optical beam. The detection means is disposed between the f.multidot..theta. lens and the optical compensation means.
In the above-mentioned optical scanning apparatuses, the synchronous detector or the detection means for adjusting the scanning starting position is arranged on a scanning beam path which does not contribute to image formation. That is, a total scanning region of the laser beam includes a region for the synchronous detector not contributing to the image formation in addition to a region contributing to the image formation on the photosensitive material. During one scanning period, accordingly, there are present not only a period contributing to the image formation (i.e., period in which an image is formed) but also a period not contributing to the image formation (i.e., period in which an image is not formed).
However, if an edge surface of the converging lens or the f.multidot..theta. lens in the scanning direction or a side surface of the deflection mirror is irradiated with a laser beam in the region not contributing to image formation, a stray light occurs, and the stray light is sometimes applied onto the photosensitive material as a light other than a light of image information. The stray light applied onto the photosensitive material causes unevenness in image density or blurring of an image, resulting in deterioration of an image quality.